1. Field of the Invention
The present invention relates to an electronic apparatus for reducing audio noise caused from unbalanced ground and mobile communication device thereof, and more particularly, to an electronic apparatus for reducing audio noise by connecting the grounding end of one circuit board of the electronic apparatus to another circuit board through a grounding wiring and by selecting the output grounding end through a selection unit.
2. Description of the Prior Art
Mobile communication is an extremely popular industry field in recent years, and all kinds of wireless communication technologies move forward constantly. Take mobile phone systems as example, e.g. global system for mobile communication (GSM), personal handy-phone system (PHS), code division multiple access (CDMA) system, wideband CDMA system, or wireless fidelity (Wi-Fi) system, which are several widely-use mobile phone systems currently.
Please refer to FIG. 1. FIG. 1 is a diagram of an electronic apparatus 10 according to the prior art. The electronic apparatus 10 is a dual mode mobile, which supports dual SIM cards. In times of the packed mobile phone stress, it is indeed a big challenge to dispose two systems within a normal size folding mobile phone. In order to reduce the size of the mobile phone, we will respectively dispose these two systems on a first circuit board 110 and on a second circuit board 120 and utilize a flexible board cable 130 to connect the first circuit board 110 and the second circuit board 120 as well as retrench pin numbers of the flexible board cable 130. In such circumstances, the pin numbers of the grounding will be minimized, which will result in unbalanced ground on both sides of the first circuit board 110 and the second circuit board 120. That is to say the reference grounding end of the first circuit board 110 is different from that of the second circuit board 120. Assume that an earphone jack 140 is disposed on the second circuit board 120, and a user inserts a receiver 150 into the earphone jack 140 to answer an incoming call through the receiver 150. In this embodiment, the receiver 150 is an earphone, which adopts a single-end audio input as its audio source and takes the reference grounding end of the second circuit board 120 as its reference point. When the user adopts the system disposed on the second circuit board 120 to use the receiver 150, therefore, the user won't hear current noises due to both the earphone jack 140 and the second circuit board 120 utilizing the same reference grounding end as their reference points. If the user adopts the system disposed on the first circuit board 110 to use the receiver 150, the user will hear a great current noise due to the reference grounding end of the earphone jack 140 being different from that of the first circuit board 110 and there being an unbalanced ground on two sides.
Please refer to FIG. 2. FIG. 2 is a diagram of sound intensity and frequency distribution of the electronic apparatus 10 in FIG. 1. Here, the electronic apparatus 10 adopts the system disposed on the first circuit board 110 (a first mode, i.e. GSM system) to use the receiver 150. As shown in FIG. 2, it can see that the sound intensity of the noises is roughly 61.4 dB at the frequency 218 Hz, the sound intensity of the noises is about 53.0 dB at the frequency 431 Hz, the sound intensity of the noises is about 46.8 dB at the frequency 650 Hz, and so on. At this time, because the reference grounding end of the receiver 150 is the grounding end of the system disposed on the second circuit board 120 (a second mode, i.e. PHS system) and the first circuit board 110 utilizes its grounding end of the GSM system as the reference point, thus the utilized reference points on two sides are different. Therefore, the user will hear a great current noise (the sound intensity of the noises falls in 45-61 dB).
Since the connectors of the flexible board cable usually have some slight DC impedance, where these slight DC impedance will cause in inconsistency between the reference grounding end of the first circuit board 110 and that of the second circuit board 120. Generally speaking, the receiver 150 adopts a single-end audio input as its audio source and takes the reference grounding end of the second circuit board 120 as its reference point. When the user adopts the system disposed on the second circuit board 120 to use the receiver 150, therefore, the user won't hear the current noises due to both the earphone jack 140 and the second circuit board 120 utilizing the same reference grounding end as their reference points. When the user adopts the system disposed on the first circuit board 110 to use the receiver 150, the user will hear a great current noise due to the reference grounding end of the earphone jack 140 being different from that of the first circuit board 110 and there being an unbalanced ground on two sides. This situation will cause a great distress to the user, and worse still, the hearing of the user will be impaired easily.